1. Field of the Invention
The present invention relates to a dry developer for developing electrostatic latent images comprising a positive charge type carrier and a negative charge type toner.
2. Description of the Prior Art
Generally, in developing images by an electrophotographic copying machine utilizing a dry developer for developing electrostatic latent images, the development is effected either by cascading over the electrostatic latent image, the carrier particles and toner particles electrostatically attracted to each other by triboelectrical charging arising from mixing and stirring of the two particles, or by rubbing against the electrostatic image, the carrier particles and toner particles arranged in the form of a magnetic brush through magnetic force.
In the above case, although the toner particles in the developer adhere to the image-formed portions by the electrostatic force of the latent image so as to be consumed thereby, the carrier particles are repeatedly used as they are without being consumed. Thus, when the developer is used for a long period, part of the toner which does not directly contribute to the developing or the so-called "spent" toner tends to be undesirably fused over the surfaces of the carrier particles, with consequent reduction in the preformance of the carrier particles which subject the toner particles to triboelectrical charging, thus resulting in adverse effects on the image quality such as reduction in density of the developed images, generation of fogging, etc. The above developer, therefore, has the disadvantage of short life and has to be replaced with a fresh one within a short time.
In order to prevent the fusion of the "spent" toner onto the surfaces of the carrier particles and prolong the life of the developer, proposals have been made such as disclosed in U.S. Pat. Nos. 3,720,617 and 4,301,228 to add metallic oxides such as silica, alumina and the like to the toner and subject the mixture to after-treatment. It has been found, however, that with the developer utilizing a negative charge type toner such a treatment results in a rise in charging amount with repeated copying, which causes reduction in density of the developed images and adhesion of the carrier particles.